The “near-infrared” (or “NIR”, the acronym for “Near InfraRed”) corresponds to the 700-1100 nm spectral band, whereas the visible light extends between 350 and 700 nm. It is sometimes considered that the near-infrared starts at 800 nm, the intermediate 700-800 nm band being eliminated using an optical filter.
The invention can be applied equally to the defense and security sectors (for example night vision) and in consumer electronics.
Conventionally, images in visible light (hereinafter called “visible images” for conciseness), generally in colors, and images in the near-infrared, are acquired independently by means of two distinct matrix sensors. In order to reduce the bulk, these two sensors can be associated with a single image-forming optical system via a dichroic beam splitter, so as to form a bi-spectral camera.
Such a configuration presents a certain number of drawbacks. Firstly, the use of two independent sensors and of a beam splitter increases the costs, the bulk, the electrical consumption and the weight of a bi-spectral camera, which is above all problematic in embedded applications, for example airborne. Furthermore, the optical system has to be specially adapted for this application, which limits the possibilities of using commercial off-the-shelf optics, even further increasing the costs.
It has also been proposed, mainly in academic-type works, to use a single matrix sensor for the acquisition of the visible and near-infrared images. Indeed, the silicon sensors commonly used in the digital cameras exhibit a sensitivity which extends from the visible to the near-infrared; this means that the cameras intended to operate only in the visible are equipped with an optical filter intended to avoid a pollution of the image by the infrared components.
The documents:                D. Kiku et al. “Simultaneously Capturing of RGB and Additional Band Images using Hybrid Color Filter Array”, Proc. of SPIE-IS&T Electronic Imaging, SPIE Vol. 9023 (2014); and        U.S. Pat. No. 8,619,143        
describe matrix sensors comprising pixels of four different types: pixels sensitive to blue light, to green light and to red light as in the conventional “RVB” sensors, but also “grey” pixels, sensitive only to the NIR radiation. Conventionally, these different types of pixels are obtained by the deposition of absorbent filters, forming a matrix of color filters, on elementary silicon sensors which are, by themselves, “panchromatic”, that is to say sensitive to all the visible and near-infrared band. Generally, the pigments used to produce these filters are transparent in the near-infrared; the images acquired by the “red”, “green” and “blue” pixels are therefore affected by the infrared component of the incident light (because the optical filter used in the conventional cameras is, obviously, absent) and a digital processing is necessary to recover colors close to reality.
The documents:                Z. Sadeghipoor et al. “Designing Color Filter Arrays for the Joint Capture of Visible and Near-Infrared images”, 16th IEEE Conference on Image Processing (2009);        Z. Sadeghipoor et al. “Correlation-Based Joint Acquisition and Demosaicing of Visible and Near-Infrared Images”, 18th IEEE Conference on Image Processing (2011)        
describe sensors having a more complex matrix of color filters, intended to optimize the reconstruction of the visible and infrared images.
Conversely, the document                Z. Sadeghipoor et al. “A Novel Compressive Sensing Approach to Simultaneously Acquire Color and Near Infrared Images on a Single Sensor” Proc. IEEE International Conference on Acoustics, Speech and Signal Processing (ICASSP), Vancouver, Canada (2013)        
describes a sensor having matrix of color filters is close to, but slightly different from, the so-called “Bayer” matrix, which is the one most commonly used in color cameras. A conventional Bayer matrix would not make it possible to separate the visible and infrared components.
These approaches use sensors in which all the pixels are equipped with a special filter. Now, in order to produce cameras with high sensitivity, it is advantageous to use sensors also comprising panchromatic pixels, without filters. In some cases, even so-called “sparse” sensors are used, comprising a high percentage of panchromatic pixels in order to pick up most of the incident radiation. These sensors exploit the fact that the chrominance of an image can be undersam pled relative to its luminance without an observer perceiving a significant degradation of its quality.
The document:
D. Hertel et al. “A low-cost VIS-NIR true color night vision video system based on a wide dynamic range CMOS imager”, IEEE Intelligent Vehicles Symposium, 2009, pages 273-278;
describes the use of a sensor comprising colored pixels and panchromatic pixels for the simultaneous acquisition of visible and near-infrared images. The method for constructing PIR images is not explained and no example of such images is shown; only “full-band” monochromatic images, that can be exploited in low-light conditions, are shown. Moreover, this article relates only to the case of an “RGBM” sensor which contains only 25% panchromatic pixels, which greatly limits the sensitivity gain that can be achieved.